In part 1 Debian Buster was installed via debootstrap. Part 2 discussed how a suitable kernel can be compiled.
Getting the LED to work
Having looked at the wrong places, I finally got the LED control to work. The DTB configures all internal devices and exposes them at /proc/devices/platform.
Aparrently there ar four PWM devices, of which only #3 is enabled. By adjusting the duty cycle one can actually control the LED:
echo 0 > /sys/devices/platform/980070d0.pwm/dutyRate3 # LED off echo 100 > /sys/devices/platform/980070d0.pwm/dutyRate3 # LED 100% brightness
